Aug-. I, 1872] 



NATURE 



267 



(Oppi-r\i:is been selected, as its conducting property is 

 better known than that of any other. The leading feature 

 of the arrangement will be comprehended by a mere 

 glance at the illustration. An equal amount of heat being 

 applied to each column, it is intended to show by the 

 elevation of the temperature of the water in the cisterns 

 c and </, what relation exists between the conductivity of 

 mercury and copper. Regarding the application of the 

 heat, it will be evident that an equal amount must in- 

 fallibly be imparted to each column if the lamp be 

 sufficiently powerful to keep the water in a state of con- 

 tinuous ebullition. Obviously the heat from the lamp, if 

 urged, will cause a rapid upward motion of the water in 

 the middle of the boiler, and a correspondingly rapid 

 descending current at each end. Accordingly lateral 

 currents varj'ingin velocity with the strength of the flame 

 applied under the boiler, will flow inwards over the upper 

 ends of the columns /and ;,>". 



Several experiments have been made under varying 

 barometric pressures and different atmospheric pressures ; 

 but the results as regards the comparative conductivity of 

 mercury and copper have proved to be very nearly alike 

 in all. The accoaipanying tables record the result of the 

 last trial, conducted as carefully as practicable. The 

 headings of the several columns explain so clearly the 

 object of the tables that it will only be necessary to state 

 that the energy inserted in the fourth column is the energy 

 developed from the beginning of the experiment. 



Referring to Table I., it will be seen that at the termi- 

 nation of four minutes from the commencement of the 

 experiment, the temperature of the water in the cistern c 

 had increased 29'o6', the differeniial temperature being 

 then 212^ - I02'56° = I09'44'. During the same period a 

 dynamic energy represented by 2'525 thermal units had 

 been transmitted past the line /•/, communicated to (i) 

 the water in the cistern ; (2) the part of the copper column 

 immersed ; (3) the metal composing the cistern ; (4) the 

 immersed part of the thermometer. But, while the entire 

 energy transmitted past the line k /, during the four 

 minutes thus amounted to only 2'525 units, the rate of 

 transmission was actually o S50 unit per minute at the 

 termination of the fourth minute. This apparent discre- 

 pancy was caused by the heat absorbed by that part of the 

 column which extends above the line /' /, the temperature 

 at the commencement of the experiment being the same 

 as that of the surrounding air, 73'5o°. Referring to 

 Table II., it will be seen that the energy transmitted 

 through the mercurial column, past the line in n, during 

 four minutes, was only o'oS/ unit against 2'525 units for 

 the copper column, although the differential temperature 

 of the water in the cistern d was I37'50" - iog"44' 

 = 2S'o6' higher than in cistern c. Accordingly, the 

 conductivity of the copper composing the column / has 



proved to be „ = 29'o6 times greater than the con- 

 ductivity of the mercury of the column i,"-, notwithstanding 

 the higher differential temperature to which the latter was 

 exposed. It will be observed that the glass, 0'02 in. 

 thick, composing the cylindrical vessel which contains the 

 mercury, will conduct some heat downward, tending to 

 increase the tcmpei'ature in the cistern d. This tendency, 

 however, will be balanced by the loss of heat occasioned 

 by the radiation of the glass cylinder, since the application 

 of the polished silver socket and the non-conducting 

 covering cannot wholly prevent the refrigerating action 

 of the surrounding air. It is important to observe, re- 

 garding the loss of heat from the latter cause, that the 

 cisterns, previous to trial, are charged with water of 

 the same temperature as the atmosphere. Now, con- 

 sidering that the increment of temperature in the cistern 

 rt' does not average more than o'4o= above that of the 

 atmosphere during the trial, it will be evident that the 

 amount of eri'or caused by radiation will be quite inap- 

 preciable, We are therefore warranted in concluding 



that the conductivity of mercury, determined by the in- 

 crement of temperature in cistern d, and by the dynamic 

 energy transmitted past the line m 11, cannot be far from 

 correct. It will be asked why columns of such small 

 diameter have been employed. The principal object 

 has been that of presenting a sectional area in the 

 mercurial column «■, corresponding as nearly as possible 

 to the size of the bulb of an ordinary thermometci-. 

 Regarding the dimensions, it will be readily admitted 

 that the conductivity of mercury might have been as- 

 certained with greater exactness, if columns of very 

 large sectional area had been employed ; but the trial 

 has conclusively established the fact that mercury 

 transmits heat from particle to panicle too slowly to 



Fu/.^ 



r/r/. 2 



effect a sufficiently rapid indication of mercurial ther- 

 mometers provided with splicrical bulbs ; and that, 

 when the heat is applied from above, the mdication ot 

 such thermometers is wholly unreliable. 



A subject of profound interest presents itself in con- 

 nection with the rate of transmission of energy exhibited 

 in the sixth column of Table I. It will be seen that 

 although the copper column/ is only o'5 in. in diameter 

 = 0'ig635 ;q. in. section, the rate of transmission at the 

 termination of the fourth minute is o'SjO unit per minute. 

 Reducing this amount to the usual standard of one square 

 foot, it will be found that the energy developed is 



-—->-, X 0350 = 623 thermal units per minute for a 



sectignal area of i §q. foot, It will be observed that this 



